This is not only OK, it is exactly why class extensions were created in the first place!
Yes, there will be a single automatically synthesized ivar and pair of getter/setter methods generated as expected.
Sorry -- missed the NSArray
vs. NSMutableArray
part. No, you can't do that; the types must be the same.
However, you don't want to return your mutable array anyway. First, the caller might modify it (a bug). But, more importantly, the caller will assume that the contents are immutable as implied by the API) and, thus, when that array's contents change out from under the caller, it may cause issue (example; caller can reasonably assume that the result of count
will be stable and can be cached).